Go to the bottom of any big dig site. (Example : smp1 /waste e) and mob hunt around there for awhile, especially at night; you will find quite a few enraged mobs. I have yet to see any mini bosses yet myself. But combine the fact I'm not out hunting them, and also there currently was a nerf on the amount they spawn; I'm not surprised

I was out the other night, and ran across an enraged creeper that met the short end of my sword...(mostly) .I find that deserts seem to be an easier 'find' (they cannot hide) and that killing everything you run across at night tends to help me run across them! Good luck!

I have seen 2 Momentus in 1 block deep water, right next to a grassland plain, if that helps. Those are the only mini-bosses I have encountered my entire time on EMC. Hope that helps you out in some way.